Combined NODDI and qMT for full-brain g-ratio mapping with complex subvoxel microstructure

Discussion: The correlation between the DTIand NODDI-based FVF was relatively high in the skeleton of the corpus callosum, where the voxels are expected to contain relatively straight, parallel white matter fibers, as required by the DTI-based FVF model (Fig.s 1 and 2). The DTI-based FVF is, however, lower than that from NODDI even in the corpus callosum, probably due to partial volume averaging with cerebrospinal fluid, or slight curvature or splay of fibers. There are voxels in which the FVF is lower than the MVF, which is not physically possible. Elsewhere in the brain, the DTI-based FVF drops significantly due to partial volume averaging of fiber orientations (Fig. 3). Simulation corroborated this observation (Fig. 4), and it indicated that the NODDI Watson model of fiber dispersion is robust to crossing fibers for the estimation of fiber volume fraction. NODDI can therefore be expected to work for all subvoxel distributions of fibers. Using NODDI, the g-ratio map for the full brain is relatively constant (Fig. 3). A flat g-ratio profile in healthy brain is expected, as the g-ratio has been shown to have an optimal value for signal conduction. g-ratio mapping in humans in vivo has the potential to be a sensitive marker of pathology in myelinated axons. Using a combination of qMT and NODDI acquisition and processing provides an imaging protocol for full-brain g-ratio computation without explicit calculation of the axon diameter distribution. Fig. 2: Maps of (top) FVF from DTI and (bottom) FVF from NODDI in the corpus callosum. Combined NODDI and qMT for full-brain g-ratio mapping with complex subvoxel microstructure Jennifer S.W. Campbell, Nikola Stikov, Robert F. Dougherty, and G. Bruce Pike McConnell Brain Imaging Centre, McGill University, Montreal, Quebec, Canada, Stanford Center for Neurobiological Imaging, Stanford University, Stanford, California, United States, Hotchkiss Brain Institute, University of Calgary, Calgary, Alberta, Canada